Aluminum-containing precipitated silicic acid having an adjustable bet/ctab ratio

ABSTRACT

The present invention relates to precipitated silica containing aluminum, which has an adjustable BET/CTAB ratio, a process for its manufacture and its use.

The present invention relates to precipitated silica containingaluminum, which has an adjustable BET/CTAB ratio, a process for itsmanufacture and its use.

The use of precipitated silicas in elastomer mixtures such as tires hasbeen known for some time. High demands are placed on silicas used intires. They should be easily dispersible in rubber, and, optionally inthe presence of coupling reagents, should have a good connection withthe polymer chains contained in rubber or the other fillers. Apart fromthe dispersibility of silica, the specific surface areas (BET or CTAB)and the oil absorption capacity (DBP) are important. The specificsurface areas are a measure for the total (BET) surface area or theouter (CTAB) surface area of silica, as both these methods utilizemolecules of varying size as adsorbate. The ratio of both these surfacearea coefficients (i.e. the BET/CTAB surface area quotient) provides anindication of the pore size distribution of the silica and the relationof “total” to “outer” surface of the silica. The surface properties ofsilicas substantially determine their possible application, or specificapplications of a silica (e.g. carrier systems or fillers for elastomermixtures) require certain surface properties.

U.S. Pat. No. 6,013,234 thus discloses the manufacture of precipitatedsilica with a BET and CTAB surface area respectively of 100 to 350 m²/g.This silica is particularly suited to incorporation in elastomermixtures, where the BET/CTAB ratios are between 1 and 1.5. EP 0 937 755discloses various precipitated silicas, which have a BET surface area ofapprox. 180 to approx. 430 m²/g and a CTAB surface area of approx. 160to 340 m²/g. These silicas are particularly suitable as carrier materialand have a BET to CTAB ratio of 1.1 to 1.3. EP 0 647 591 discloses aprecipitated silica, which has a ratio of BET to CTAB surface area of0.8 to 1.1, whereby these surface coefficients can take on absolutevalues of up to 350 m²/g. EP 0 643 015 discloses a precipitated silica,which can be used as abrasive and/or thickening components in toothpastes, which has a BET surface area of 10 to 130 m²/g and a CTABsurface area of 10 to 70 m²/g, i.e. a BET to CTAB ratio of approx. 1 to5.21.

Precipitated silica containing aluminum as filler are frequently used inthe manufacture of tires.

Thus, EP 0 983 966 discloses a precipitated silica containing aluminumwith the following physicochemical properties: BET surface area 80-180m²/g CTAB surface area 80-139 m²/g DBP number 100-320 g/100 g Al₂O₃content <5%.

Precipitated silicas of this kind can be improved with respect to theiruse as elastomer filler.

It was discovered that a precipitated silica containing aluminum with ahigh BET surface area is particularly well suited as a filler (e.g. fortires).

The object of the present invention is therefore precipitated silicas,which have BET surface area in the range 150-400 m²/g, preferably190-300 m²/g CTAB surface area in the range 140-350 m²/g, preferably145-250 m²/g; 145-200 m²/g Al₂O₃ content in the range 0.2-5% by weight,preferably 1-3% by weight.

The preferred areas can each be adjusted independently of one another.

The precipitated silicas according to the present invention preferablyhave a specific ratio of BET to CTAB surface area. The BET/CTAB ratiocan be in the following ranges: 1.0-1.6, preferably 1.2-1.6.

In addition, the precipitated silicas can be characterized by a wkcoefficient (ratio of the peak level of particle size distribution ofthe particles not degradable by ultrasound in the size range 1.0-100 μmto the peak level of the degraded particles in the size range <1.0 μm)of ≦3.4, preferably 0.1 to 3.4, particularly preferably 0.1 to 3.0and/or by a DBP absorption of 180-320 g/100 g, in which case the silicasin a first embodiment of the present invention have a DBP in thepreferred ranges 200-320 g/100 g; 250-320 g/100 g and 250-300 g/100 gand in a further embodiment of the present invention have a DBP with thepreferred range 180-300 g/100 g and 180-250 g/100 g.

Known precipitated silicas have clearly higher wk coefficients and/ormaximums displaced to other values in the particle size distributions.

It has been shown that the wk coefficient is a measure for thedispersibility of a precipitated silica, as it is a measure for thedecomposability (dispersibility) of the precipitated silica. Aprecipitated silica is all the more easily dispersible the smaller thewk coefficient is, i.e. the more particles are decomposed whenincorporated into rubber.

The BET or CTAB surface areas or their ratio of the precipitated silicaaccording to the present invention are preferably in the followingareas: BET CTAB BET/CTAB [m²/g] [m²/g] ratio 195 145 1.34 200 150 1.33210 149 1.41 280 147 1.90 315 148 2.13 350 150 2.33 370 152 2.43

The precipitated silicas according to the present invention have surfaceproperties, which make them particularly well suited as filler forelastomers. This can be ascertained by the modified Sears number V₂,which here is preferably between 5 and 35 ml/5 g, particularlypreferably between 20 and 30 ml/5 g.

Another object of the present invention is a process for manufacture ofa precipitated silica with a BET surface area in the range 150-400 m²/gCTAB surface area in the range 140-350 m²/g Al₂O₃ content in the range0.2-5% by weight,where

-   a) an aqueous water glass solution is filled into a vessel-   b) water glass and acidifier are metered into this vessel with    stirring at 55-95° C. for 30-100 minutes simultaneously,-   c) acidified with acidifier to a pH value of approx. 5 and-   d) filtered and dried,-    on the condition that aluminum compounds are added in steps b)    and/or c).

The silicas manufactured by the process according to the presentinvention have the above-mentioned preferred ranges for the parametersBET, CTAB, DBP, Al₂O₃ content and Sears number.

The water glass solution introduced in step a) can have the sameconcentration as the water glass used in step b) (e.g. density 1.34%,27.4%, SiO₂, 8.1% Na₂O). Diluted solutions can also be used, e.g.0.5-10% SiO₂ and correspondingly 0.15%-3% Na₂O.

The constituents added in steps b) and c), i.e. water glass andacidifier can each have identical or different concentrations and/orfeed rates. In a procedural variant the concentration of theconstituents used in both steps is the same, though the feed rate of theconstituents in step c) is 125-140% of the feed rate in step b). Inanother variant the feed rate in step c) is only 30-100, preferably50-80% of that in step b).

Apart from water glass (sodium silicate solution) other silicates suchas potassium silicate can also be used. Sulfuric acid can preferably beused as acidifier, but other acidifiers such as HCl, HNO₃, H₃PO₄,CH₃COOH, or CO₂ can also be used.

Aluminum compounds can be added in both steps b) and c), but also onlyin one of steps b) or c) in each case identical or different as a solid,aqueous solution or as acidifier/aluminum compound mixed solution.

The aluminum compounds can be used as aqueous solutions of preferablyAl₂(SO₄)₃, but e.g. also Al(NO₃)₃, AlCl₃ or Al(OAc)₃ with aconcentration of 50-130 g/l, preferably 70-110 g/l in water.Alternatively, acidifier/aluminum compound mixed solutions can be used.

Filtration and drying of the silicas according to the present inventionare known to the expert and can be gleaned from e.g. the above-mentionedpatent documents. Silicic acid according to the present invention ispreferably dried by short-term drying such as e.g. spray drying(optionally in the spray nozzle dryer), flash and/or spin flash dryer.Spray drying can be carried out e.g. according to U.S. Pat. No.4,097,771. Here precipitated silica is produced in the spray nozzledryer, which is obtained in particle form having an average diameter ofover 80, in particular over 90, particularly preferably over 200 μm.

After drying, if required grinding and/or granulating with/without aroller compactor can be performed. In this case the average diameter ofthe end product after granulation is ≧1 mm.

The silicas according to the present invention can accordingly be usede.g. as fillers in elastomer mixtures, vulcanizable rubber mixtures,other vulcanizates in particular for tires, battery separators,anti-blocking agents, matting agents in paints, paper coatings,defoamers, in seals, keypads, conveyor belts and/or window seals.

The silica according to the present invention can be optionally modifiedwith organosilicon compounds (silanes) of formulae I to III[R¹ _(n)(RO)_(r)Si(Alk)_(m)(Ar)_(p)]_(q)[B]  (I),R¹ _(n)(RO)_(3-n)Si(Alkyl)  (II),orR¹ _(n)(RO)_(3-n)Si(Alkenyl)  (III),with the following meanings

-   B: —SCN, —SH, —SC(O)CH₃, —SC(O)(CH₂)₆CH₃, —Cl, —NH₂, —OC(O)CHCH₂,    —OC(O)C(CH₃)CH₂ (if q=1), or —S_(x)— (if q=2),-   R and R¹: an aliphatic, olefinic, aromatic or aryl aromatic radical    with 2 to 30 C atoms, which can optionally be substituted by the    following groups: hydroxy, amino, alcoholate, cyanide, thiocyanide,    halogen, sulfonic acid, sulfonic acid ester, thiol, benzoic acid,    benzoic acid ester, carbonic acid, carbonic acid ester, acrylate,    methacrylate, organosilane radical, where R and R¹ can have an    identical or different meaning or substitution,-   n: 0; 1 or 2,-   alk: a divalent unbranched or branched hydrocarbon radical with 1 to    6 carbon atoms,-   m: 0 or 1,-   ar: an aryl radical with 6 to 12 C atoms, preferably 6 C atoms,    which can be substituted by the following groups: hydroxy, amino,    alcoholate, cyanide, thiocyanide, halogen, sulfonic acid, sulfonic    acid ester, thiol, benzoic acid, benzoic acid ester, carbonic acid,    carbonic acid ester, organosilane radical,-   p: 0 or 1 with the proviso that p and n do not simultaneously mean    0,-   x: a number from 2 to 8,-   r: 1, 2 or 3, with the proviso that r+n+m+p=4,-   alkyl: a monovalent unbranched or branched unsaturated hydrocarbon    radical with 1 to 20 carbon atoms, preferably 2 to 8 carbon atoms,-   alkenyl: a monovalent unbranched or branched unsaturated hydrocarbon    radical with 2 to 20 carbon atoms, preferably 2 to 8 carbon atoms.

The silica according to the present invention can also be modified withorganosilicone compounds having the composition R² _(4-n)SiX_(n) (withn=1, 2, 3), [SiR² _(x)X_(y)O]_(z) (with 0≦x≦2; 0≦y≦2; 3≦z≦10, withx+y=2), [SiR² _(x)X_(y)N]_(z) (with x+y=2), SiR² _(n)X_(m)OSiR²_(o)X_(p) (with 0≦n≦3; 0≦m≦3; 0≦o≦3; 0≦p≦3, with n+m=3, o+p=3), SiR²_(n)X_(m)NSiR² _(o)X_(p) (with 0≦n≦3; 0≦m≦3; 0≦o≦3; 0≦p≦3, with n+m=3,o+p=3), SiR² _(n)X_(m)[SiR² _(x)X_(y)O]_(z)SiR² _(o)X_(p) (with 0≦n≦3;0≦m≦3; 0≦x≦2; 0≦y≦2; 0≦o≦3; 0≦p≦3; 1≦z≦10000, with n+m=3, x+y=2, o+p=3).These compounds can be linear, cyclic and branched silane, silazane andsiloxane compounds. R² can be alkyl and/or aryl radicals with 1 to 20carbon atoms, which can be substituted by functional groups such as thehydroxy group, the amino group, polyethers, such as ethylene oxideand/or propylene oxide, and halogenide groups, such as fluoride. R² mayalso contain groups such as alkoxy, alkenyl, alkinyl and aryl groups andsulfurous groups. X can be reactive groups such as silanole, amino,thiol, halogenide, alkoxy, alkenyl and hydride groups.

Linear polysiloxanes having the composition SiR² _(n)X_(m)[SiR²_(x)X_(y)O]_(z)SiR² _(o)X_(p) (with 0≦n≦3; 0≦m≦3; 0≦x≦2; 0≦y≦2; 0≦o≦3;0≦p≦3; 1≦z≦10000, with n+m=3; x+y=2; o+p=3) are preferably used, inwhich R² is preferably represented by a methyl group.

Polysiloxanes having the composition SiR² _(n)X_(m)[SiR²_(x)X_(y)O]_(z)SiR² _(o)X_(p) (with 0≦n≦3; 0≦m≦1; 0≦x≦2; 0≦y≦2; 0≦o≦3;0≦p≦1; 1≦z≦1000, with n+m=3, x+y=2, o+p=3) are particularly preferablyused, in which R² is preferably represented by methyl.

Modifying the optionally granulated, ungranulated, ground and/orunground precipitated silica with one or more of the above-mentionedorganosilanes can be undertaken in mixtures of 0.5 to 50 parts, relativeto 100 parts precipitated silica, in particular 1 to 15 parts, relativeto 100 parts precipitated silica, whereby reaction between precipitatedsilica and organosilane can be carried out during production of themixture (in situ) or outside of production by spraying and subsequenttempering of the mixture, by mixing of the organosilane and the silicasuspension with subsequent drying and tempering (for example accordingto DE 34 37 473 and DE 196 09 619) or in accordance with the processdescribed in DE 196 09 619 or DE-PS 40 04 781.

All bifunctional silanes, which can on the one hand effect coupling tothe filler containing silanole groups and on the other hand coupling tothe polymer, are basically suitable as organosilicon compounds. Theusual quantities of organosilicon compounds are 1 to 10% by weight,relative to the total quantity of precipitated silica.

Examples for these organosilicon compounds are:Bis(3-triethoxysilylpropyl)tetrasulfide,bis(3-triethoxysilylpropyl)disulfide, vinyltrimethoxysilane,vinyltriethoxysilane, 3-mercaptopropyltrimethoxysilane,3-mercaptopropyltriethoxy silane, 3-aminopropyltrimethoxysilane,3-aminopropyltriethoxysilane. Other organosilicon compounds aredescribed in WO 99/09036, EP 1 108 231, DE 101 37 809, DE 101 63 945, DE102 23 658.

In a preferred embodiment of the inventionbis(3-triethoxysilyl-propyl)tetrasulfide andbis(3-triethoxysilyl-propyl)disulfide can be used as silane.

Use of the Silica According to the Present Invention in ElastomerMixtures

The silica according to the present invention can be mixed intoelastomer mixtures, tires or vulcanizable rubber mixtures as areinforcing filler in quantities of 5 to 200 parts, relative to 100parts per rubber as powder, micropearls or granulate and with silanemodification or without silane modification.

One or more of the above-mentioned silanes can be added to theelastomers along with the silicas, in which case the reaction betweenfiller and silane occurs during the mixing procedure at increasedtemperatures (in-situ modification) or in already premodified form (forexample DE-PS 40 04 781), that is, both reaction partners are reactedoutside the actual mixture production.

Apart from mixtures, which contain exclusively the silicas according tothe present invention, with and without organosilanes according toformulae I to III as fillers, the elastomers can additionally be filledwith one or more more or less reinforcing fillers. In the first instancea blend between carbon blacks would be useful here (for example furnace,gas, flame, acetylene carbon blacks) and the silicas according to thepresent invention, with and without silane, but also between naturalfillers, such as for example clays, siliceous chalk, other commercialsilicas and the silicas according to the present invention.

The blend ratio here also, as with metering of the organosilanes, isaligned with the properties to be achieved in the finished rubbercompound. A ratio of 5-95% between the silicas according to the presentinvention and the other above-mentioned fillers is conceivable and isalso realized within this framework.

Apart from the silicas according to the present invention, theorganosilanes and other fillers, the elastomers form another importantconstituent of the rubber mixture. The silicas according to the presentinvention can be used in all elastomers that can be cross-linked withaccelerators/sulfur, but also with peroxides. Examples of these areelastomers, natural and synthetic, oil-extended or not, as singlepolymers or blend with other rubbers, such as for example naturalrubbers, butadiene rubbers, isoprene rubbers, butadiene styrene rubbers,in particular SBR, produced by means of the solution polymerizationprocess, butadiene acrylnitrile rubbers, butyl rubbers, terpolymers madeof ethylene, propylene and non-conjugated dienes. The followingadditional rubbers also come into consideration for rubber mixtures withthe above-mentioned rubbers: carboxyl rubbers, epoxide rubbers,trans-polypentenamer, halogenated butyl rubbers, rubbers from2-chlorobutadiene, ethylene vinyl acetate copolymers, ethylene propylenecopolymers, and optionally chemical derivatives of natural rubber aswell as modified natural rubbers.

Likewise known are other constituents such as plasticizers, stabilizers,activators, pigments, antioxidants and processing aids in the usualdosages.

The silicas according to the present invention, with and without silane,are utilized in all rubber applications, such as for example tires,conveyor belts, seals, V-belts, hoses, shoe soles, etc.

A further object of the invention is elastomer mixtures, in particularvulcanizable rubber mixtures, which contain the silicas according to thepresent invention in quantities of 5 to 200 parts, relative to 100 partselastomer or rubber. Processing this silica and manufacturing themixtures containing this silica are carried out in a manner customary tothe rubber industry on a internal mixer or open-roll mill. The availableform or form of use can be both powder, micropearls or granulate. Heretoo the silicas according to the present invention do not differ fromthe known white fillers.

To achieve a good set of values in a polymer mixture, dispersion of theprecipitated silica in the matrix, the polymer, is very decisive.

Use of the Precipitated Silicas According to the Present Invention inPaper Coatings

Inks currently in use, which above all are used in all types ofso-called inkjet printing and its related processes, are mostly anionicin nature. Therefore, with respect to fixing of the coloring agent (ofdyes and/or pigments), the color brilliance, the print sharpness andprint depth it is of considerable significance that the media to beprinted have on their surface, or in their surface regions, particleswith an at least partial cationic surface.

Silicas and silicates today are often used for the above-mentionedcoating formulations (e.g. paper, film coating). Modification of thesesilicas and silicates of such a type where active, i.e. accessible,cationic sites (EP 0 492 263) occur on their surface, complies withcurrent requirements on account of the frequently used anionic coloringagents.

Due to the influence of the inbuilt metallic ions on the refractionindex additional advantages can arise with respect to use in transparentmedia, such as e.g. with the use of silicas/silicates in coatings forfoils.

The object of the invention is therefore also the use of theprecipitated silica according to the present invention, or theprecipitated silica manufactured via the process according to thepresent invention as an additive in paper manufacture or in papercoatings.

In particular precipitated silicas according to the present inventioncan be used in paper coatings of e.g. inkjet papers and in coatings forother printed media, such as e.g. overhead films or printable textiles.

The precipitated silicas according to the present invention can be putto use not only as dried and optionally ground products, but also asdispersions. Advantages in further processing or cost advantages canespecially be in using dispersed filter cakes of the precipitatedsilicas according to the present invention for application in paper pulpor in coatings of printable media.

For use in paper manufacture it is possible to mix in auxiliary agents,which are common in the paper industry, such as e.g. polyalcohols,polyvinyl alcohol, synthetic or natural polymers, pigments (TiO₂, Feoxides, Al metal filters), but also undoped silicas, i.e. withoutaluminum additive (precipitated silicas or aerosils) with thedispersions of the precipitated silicas according to the presentinvention.

The physicochemical data of the precipitated silicas according to thepresent invention are determined by the following methods:

-   BET surface area Areameter, Ströhlein, according to ISO 5794/Annex D-   CTAB surface area at pH 9, according to Janzen and Kraus in Rubber    Chemistry and Technology 44 (1971) 1287    Determining the Solids Content of Silica Suspensions

The silica suspension (e.g. slurry) is dried in the IR dryer to aconstant weight. The drying loss generally predominantly comprises watermoisture and only traces of other volatile constituents.

Execution:

2.0 g silica suspension are filled into a previously tared aluminum dishand the cover of the IR dryer unit (Mettler, type LP 16) is closed. Oncethe start key is pushed drying of the suspension begins at 105° C.,which is automatically terminated when the decrease in weight per timeunit drops below a value of 2 mg/120 s. The drying loss in % isindicated directly by the device on selection of the 0-100% mode.Measuring is performed as repeat determination.

Determining the Moistness of Silicas

According to this method following ISO 787-2 the volatile portions(herein referred to as moisture for simplicity) of silica are determinedafter 2 hours of drying at 105° C. This drying loss generallypredominantly comprises water moisture.

Execution

10 g of the powder, spherical or granular silica are weighed preciselyto 0.1 mg (weighed sample E) into a dry weighing bottle with groundglass cover (diameter 8 cm, height 3 cm). The sample is dried in adrying cabinet with the top open for 2 h at 105±2° C. Next the weighingbottle is sealed and cooled to room temperature in a desiccator cabinetwith silica gel as a drying agent. The weighed portion A is determinedgravimetrically.

The moisture is determined in % according to (E in g−A in g)×100%/E ing.

Measuring is performed as a repeat determination.

Determining DBP Absorption

The DBP absorption (DBP number), which is a measure for the absorptivecapacity of the precipitated silica, is determined in accordance withDIN standard 53601 as follows

Execution

12.50 g powder or spherical silica with 0-10% moisture content(optionally the moisture content is adjusted by drying at 105° C. in thedrying cabinet) are added to the kneading chamber (article number 279061) of the Brabender Absorptometer “E”. In the case of granulates thescreened fraction of 3.15 to 1 mm (stainless steel screen by Retsch) isused (by gently pressing the granulates through the screen with 3.15 mmpore size using a plastic spatula). With constant stirring (rotationalspeed of kneader blades 125 rpm) dibutylphthalate is dropped through the“Dosimaten Brabender T 90/50” at a rate of 4 ml/min into the mixture, atroom temperature. Mixing takes place with only minimal power consumptionand is observed with reference to the digital display. Towards the endof the determination the mixture becomes pasty, which is indicated bymeans of a steep rise in the power consumption. When 600 digits aredisplayed (torque of 0.6 Nm) both the kneader and also the DBP meteringare switched off by an electrical contact. The synchronous motor for DBPsupply is coupled to a digital counter, so that DBP use can be read offin ml.

Evaluation

DBP absorption is indicated in g/100 g and calculated using thefollowing formula from measured DBP usage. The density of DBP at 20° C.is typically 1.047 g/ml.DBP absorption in g/100 g=DBP usage in ml×density of DBP ing/ml×100/12.5 g.

DBP absorption is defined for water-free, dried silica. When moistprecipitated silicas are used the value is to be corrected by means ofthe following correction table.

The corrected value corresponding to the water content is added to theexperimentally determined DBP value; e.g. a water content of 5.8% wouldmean the addition of 33 g/100 g for DBP absorption. Correction table fordibutylphthalate absorption-water-free- .% water % water .0 .2 .4 .6 .80 0 2 4 5 7 1 9 10 12 13 15 2 16 18 19 20 22 3 23 24 26 27 28 4 28 29 2930 31 5 31 32 32 33 33 6 34 34 35 35 36 7 36 37 38 38 39 8 39 40 40 4141 9 42 43 43 44 44 10 45 45 46 46 47Determining wk Coefficients: Aggregate Size Distribution by Means ofLaser DiffractionSample Preparation

If the silica to be determined is a granulate, then 5 g of the granularsilica are added to a beaker and the coarse granulate pieces are crushedwith a pestle but not pounded. 1.00 g of the crushed, powder orspherical silica with 5±1% moisture content (optionally the moisturecontent is adjusted by drying at 105° C. in the drying chamber oruniform humidifying), which was manufactured no more than 10 daysbeforehand, is weighed into a 30 ml centrifuge tube with a convex base(height 7 cm, Ø 3 cm, depth of convex bulging 1 cm) and mixed with 20.0ml dispersion solution (hydrophilic silicas: 20.0 g sodiumhexametaphosphate (by Baker) filled to 1000 ml with deionized water;hydrophobic silicas: 200.0 ml reagent grade ethanol with 2.0 mlconcentrated ammonia solution and 0.50 g Triton X-100 (by Merck) filledto 1000 ml with deionized water). Then the centrifuge tube is placedinto a double-walled glass cooling vessel (80 ml volumetric capacity,height 9 cm, Ø 3.4 cm) with cold water connections for tap water (20°C.) and the sample is treated for 270 s with an ultrasound finger (byBandelin, type UW 2200 with Horn DH 13 G and diamond plate Ø 13 mm). Forthis 50% power and 80% pulse (corresponds to 0.8 s power and 0.2 spause) is set on the power supply unit (Sonopuls, by Bandelin, type HD2200) of the ultrasound finger. Heating of the suspension is adjusted bywater cooling to maximum <8° C. As soon as the sample is added to theliquid module of the laser diffraction unit within 15 min, thesuspension is stirred with a magnetic stirrer to prevent possiblesedimentation.

Execution

Prior to measuring, the laser diffraction unit LS 230 (by Coulter) andthe liquid module (LS Variable Speed Fluid Module Plus with integratedultrasound finger CV 181, by Coulter) is left to run warm for 2 h andthe module (menu “Control/rinse”) is rinsed for 10 min.

In the task bar of the unit software the menu feature “Measurement” isused to select the file window “Calculate Opt. Model” and the refractionindices are defined in a .rtf file as follows: fluid refraction index B.I. Real=1.332; Material refraction index Real=1.46; Imaginary=0.1.

In the file window “Measuring cycle” the output of the pump speed is setto 26% and the ultrasound output of the integrated ultrasound finger CV181 is set to 3. The ultrasound features “during sample adding”, “10seconds before each measurement” and “during measuring” are to beactivated. In addition, the following features are selected in this filewindow:

set offset measurement, adjustment, background measurement, measurementconcentration, input sample info, input measurement info, start 2measurements, autom. rinse, with PIDS data.

On completion of calibration measuring with an LS Size Control G15Standard (by Coulter) and background measuring samples are added.Suspended silica is added until such time as light absorption of 45-55%is achieved and the unit displays “OK”.

Measuring is done at room temperature with the evaluation model of the.rtf file defined hereinabove. Three repeated tests each of 60 secondswith a wait time of 0 seconds of each silica sample are carried out.

From the raw data curve the software calculates the particle sizedistribution on the basis of volume distribution, taking intoconsideration the Mie theory and the optical model by Fraunhofer.Typically, a bimodal distribution curve is found with an A mode between0-1 μm (maximum at approx. 0.2 μm) and a B mode between 1-100 μm(maximum at approx. 5 μm). According to FIG. 1 the wk coefficient can bedetermined from this and is given as an average value of six individualmeasurements.

An essential point here is that the energy input via ultrasoundrepresents a simulation of the energy input via mechanical forces inindustrial mixers in the tire industry.

FIG. 1 is a schematic illustration of the values required to calculatethe wk coefficients.

The curves show a first maximum in particle size distribution in therange um 1.0-100 μm and show another maximum in the range <1.0 μm. Thepeak in the range 1.0-100 μm gives the portion of uncomminuted silicaparticles after ultrasound treatment. These fairly coarse particles arepoorly dispersed in the rubber mixtures. The second peak with muchsmaller particle sizes (<1.0 μm) indicates that portion of silicaparticles which has been comminuted during ultrasound treatment. Thesevery small particles are very well dispersed in rubber mixtures.

The wk coefficient is now the ratio of the peak level of thenon-degradable particles (B), whose maximum is in the range 1.0-100 μm(B′), to the peak level of the degraded particles (A), whose maximum isin the range <1.0 μm (A′).

Determining the Modified Sears Number of Silicas

The modified Sears number (hereinafter referred to as Sears number V₂)as measure for the number of free hydroxyl groups can be determinedthrough titration of silica with potassium hydroxide solution in therange of pH 6 to pH 9.

The following chemical reactions form the basis of the determinationmethod, where “Si”—OH is used to symbolize a silanole group:“Si”—OH+NaCl “Si”—ONa+HClHCl+KOH KCl+H₂O.Execution

10.00 g of a powder, spherical or granular silica with 5±1% moisture areground for 60 seconds in the IKA M 20 universal mill (550 W; 20 000rpm). If required, the moisture content must be adjusted by drying at105° C. in the drying chamber or by even humidifying. 2.50 g of thesilica thus treated are weighed at room temperature into a 250 mltitrating vessel and mixed with 60.0 ml reagent grade methanol. Afterthe sample is fully perfused 40.0 ml deionized water are added and thewhole is dispersed by means of Ultra Turrax T 25 (KV-18G agitator shaft,18 mm diameter) for 30 seconds at a speed of 18 000 rpm. The sampleparticles adhering to the vessel edge and stirrer in the suspension arerinsed with 100 ml deionized water and tempered in a thermostatic waterbath to 25° C.

The pH meter (by Knick, type: 766 Calimatic pH meter with temperaturesensor) and the pH electrode (single-rod measuring chain by Schott, typeN7680) are calibrated at room temperature using buffer solutions (pH7.00 and 9.00). First, the starting pH value of the suspension at 25° C.is measured with the pH meter, and then depending on the outcome, it isadjusted with a potassium hydroxide solution (0.1 mol/l) or hydrochloricacid solution (0.1 mol/l) to a pH value of 6.00. The KOH or HCl solutionconsumption in ml up to pH 6.00 corresponds to V₁′.

After this 20.0 ml sodium chloride solution (250.00 g reagent gradeNaCl. filled with deionized water to 1 l) are added. Titration to pH9.00 is continued with 0.1 mol/l KOH. The KOH solution consumption in mlup to pH 9.00 corresponds to V₂′.

Next the volumes V₁′, or V₂′ are first standardized to the theoreticalweighed sample of 1 g and supplemented with five, resulting in V₁ andSears number V₂ in ml/5 g units. The measurements are each performed asrepeat determinations.

EXAMPLES Example 1

51.5 l water and 3.8 l water glass (density 1.346 kg/l, 27.4% SiO₂, 8.1%Na₂O) are introduced into a reactor made of high-quality stainless steelwith propeller stirring gear and double shell heating.

Then 8.2 l/h water glass, 0.345 l/h aluminum sulfate solution (110 g/lAl₂O₃) as well as 0.6 l/h sulfuric acid (96%, density 1.84 kg/l) aremetered with vigorous stirring at 87° C. for 80 minutes. On completionof the preset metering period the supply of water glass and aluminumsulfate solution is stopped and more sulfuric acid is added, until a pH(measured to suspension tempered to 20° C.) of 5.0 is reached.

The resulting suspension is filtered as usual and washed with water to asodium sulfate content <4% by weight. The filter cake is liquefied withaqueous sulfuric acid and a shearing unit. The silica slurry with 16%solids content is then spray-dried.

The resulting powder product has a BET surface area of 195 m²/g and aCTAB surface area of 145 m²/g, a DBP absorption of 306 g/100 g as wellas a wk coefficient of 1.46. The aluminum oxide content of the endproduct is 1.0% and the Sears number V₂ is 25.7 ml/5 g.

Example 2

51.5 l water and 3.8 l water glass (density 1.346 kg/l, 27.4% SiO₂, 8.1%Na₂O) are introduced into a reactor made of high-quality stainless steelwith propeller stirring gear and double shell heating.

Then 8.2 l/h water glass, 0.865 l/h aluminum sulfate solution (110 g/lAl₂O₃) as well as 0.475 l/h sulfuric acid (96%, density 1.84 kg/l) aremetered with vigorous stirring at 85° C. for 80 minutes. On completionof the specified metering period the supply of water glass and aluminumsulfate solution is stopped and more sulfuric acid is added, until a pH(measured to suspension tempered to 20° C.) of 5.0 is reached.

The resulting suspension is filtered as usual and washed with water to asodium sulfate content <4% by weight. The filter cake is liquefied withaqueous sulfuric acid and a shearing unit. The silica slurry with 16%solids content is then spray-dried.

The resulting powder product has a BET surface area of 200 m²/g and aCTAB surface area of 150 m²/g, a DBP absorption of 285 g/100 g as wellas a wk coefficient of 2.78. The aluminum oxide content of the endproduct is 2.0% and the Sears number V₂ is 23.2 ml/5 g.

Example 3

51.5 l water and 3.8 l water glass (density 1.346 kg/l, 27.4% SiO₂, 8.1%Na₂O) are introduced into a reactor made of high-quality stainless steelwith propeller stirring gear and double shell heating.

Then 8.2 l/h water glass, 2.170 l/h aluminum sulfate solution (110 g/lAl₂O₃) as well as 0.185 l/h sulfuric acid (96%, density 1.84 kg/l) aremetered with vigorous stirring at 83° C. for 80 minutes. On completionof the preset metering period the supply of water glass and aluminumsulfate solution is stopped and more sulfuric acid is added at a flowrate of 0.475 l/h, until a pH (measured to suspension tempered to 20°C.) of 5.0 is reached.

The resulting suspension is filtered as usual and washed with water to asodium sulfate content <4% by weight. The filter cake is liquefied withaqueous sulfuric acid and a shearing unit. The silica slurry with 18%solids content is then spray-dried.

The resulting powder product has a BET surface area of 210 m²/g and aCTAB surface area of 149 M²/g, a DBP absorption of 247 g/100 g as wellas a wk coefficient of 3.11. The aluminum oxide content of the endproduct is 4.5% and the Sears number V₂ is 25.7 ml/5 g.

Example 4

In following example the following substances are used: Krynol 1712Styrene butadiene rubber based on emulsion polymerization X 50 S 50:50blend of Si 69 (bis(3- triethoxysilylpropyl)tetrasulfane and N 330(carbon black, commercial product by Degussa AG) ZnO zinc oxide Stearicacid Naftolene aromatic oil Lipoxol 4000 polyethylene glycol Vulkanox4020 N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylendiamine DPGdiphenylguanidine CBS N-cyclohexyl-2-benzthiazylsulfenamide Sulfur

The precipitated silicas according to the present invention are mixed inas powder in comparison to the standard silica Ultrasil VN2 GR (DegussaAG) in a pure E-SBR mixture (data in phr) Krynol 1712 137.5 Silica 50 X50 S 3 ZnO 3 Stearic acid 1 Vulkanox 4020 2 Lipoxol 4000 1.5 DPG 1.5 CBS1.5 Sulfur 2.2

A Werner & Pfleiderer 1.5 N type mixer was used, at 45 min⁻¹, at 1:1.11friction, at a ram pressure of 5.5 bar, a void fraction of 1.6 l, afilling level of 0.73 and a throughflow temperature of 90° C. The mixingprocess used was a 3-step method: step 1 0-1 min polymers, 1-2 minadditional constituents apart from accelerator and sulfur, cleanse for 2min, mix for 2-5 min and cleanse (from 3 min mix at 70 min⁻¹),discharge. The mixture is then stored for 24 h at room temperature. Step2: 0-1 min batch step 1 plasticize at 70 min⁻¹ and 0.71 filling level,1-3 min maintain batch temperature of 150° C. by speed variation, 3 mindischarge, store for 4 h at room temperature. Step 3: 0-2 min batch step2, mix accelerator and sulfur at 40 min⁻¹ and 50° C. throughflowtemperature and filling level 0.69, discharge after 2 min and form aband on a laboratory mixing roller (diameter 20 mm, length 450 mm,throughflow temperature 50° C.), homogenize through 3× right and shear3× left, tumble 3× with wide (3.5 mm) and 3× with narrow (1 mm) rollnip, discharge band.

Vulcameter testing at 160° C. was performed in accordance with DIN53529/2 or ISO 6502, voltage moduli and elongation at break weredetermined in accordance with DIN 53504, Shore hardness in accordancewith DIN 53 505 at 23° C., ball rebound in accordance with ASTM D 5308,heat build up in accordance with ASTM D 623 A (‘0.175 inch, stroke, 25min), MTS data in accordance with ASTM D 2231-87 (10 Hz, 10%prestrained, Ampl. Sweep: 0.15-7%), dispersion coefficient determined bymeans of surface topography [A. Wehmeier, “Filler Dispersion Analysis byTopography Measurements”, Technical Report TR 820, Degussa AG, AppliedTechnology Advanced Fillers]. Silica Silica according according to toUltrasil invention, invention, VN2 GR Ex. 1 Ex. 2 ML(1 + 4) at 100° C.;2nd [ME] 51 52 56 MDR: 160° C.; 0.5° t 90% [min] 9.9 9.6 9.8 t 80%-t 20%[min] 2.7 2.6 2.6 Vulcanizate data Module 100% [MPa] 1.1 1.3 1.3 Module300% [MPa] 4.6 5.3 5 Strain at break [%] 470 450 420 Shore A hardness[SH] 52 52 52 Ball Rebound, 0° C. [%] 19.3 19.5 20.5 Ball Rebound, 60°C. [%] 65 65.4 67.8 Goodrich Flexometer, 0.225 inch, 25 min, RT HeatBuild Up [° C.] 79 79 79 Permanent Set [%] 1.8 1.7 1.7 MTS, 16 Hz, 50N+/− 25N E*, 0° C. [MPa] 7.7 8.4 8.3 E*, 60° C. [MPa] 5.0 5.2 5.2 tan d,0° C. [−] 0.282 0.277 0.263 tan d, 60° C. [−] 0.100 0.102 0.100Dispersion, topography Peak area [%] 3.9 1.8 2.6

Compared to the standard silica Ultrasil VN2 GR the silicas 1 and 2,according to the present invention, result in higher modulus values,higher elongations at break, higher E* values and clearly improveddispersion (corresponding to better abrasion resistance). Moreover bothsilicas according to the present invention exhibit a uniform heat buildup in spite of the higher surface area compared to Ultrasil VN2 GR. Thiscorresponds to equally good heating behavior under dynamic stress, fromwhich an equally high service life of the elastomer mixture under stressis derived.

1. A precipitated silica comprising a BET surface area of 150-400 m²/g aCTAB surface area of 145-350 m²/g an Al₂O₃ content of 0.2-5% by weightand a modified Sears number V₂ of 5-35 ml/(5 g).
 2. A precipitatedsilica of claim 1, wherein the precipitated silica has a DBP absorptionof from 180 to 320 g/100 g.
 3. A precipitated silica of claim 1, whereinthe precipitated silica has a BET/CTAB surface ratio of from 1.0 to 1.6.4. A precipitated silica of claim 3 wherein the precipitated silica hasa BET/CTAB surface ratio of from 1.2 to 1.6.
 5. A precipitated silica ofclaim 1, wherein the precipitated silica has a BET/CTAB surface ratio offrom 1.33 to 2.43.
 6. A precipitated silica of claim 1, wherein theprecipitated silica has a wk coefficient ≦3.4.
 7. A precipitated silicaof claim 1, wherein the precipitated silica surface has been modifiedwith organosilanes of the formulae[R¹ _(n)(RO)_(r)Si(Alk)_(m)(Ar)_(p)]_(q)[B]  (I)R¹ _(n)(RO)_(3-n)Si (Alkyl)  (II)orR¹ _(n)(RO)_(3-n)Si (Alkenyl)  (III) in which B is —SCN, —SH, —SC(O)CH₃,—SC(O) (CH₂)₆CH₃, —Cl, —NH₂, —OC(O)CHCH₂, —OC(O)C(CH₃)CH₂ (if q=1), or—S_(x)— (if q=2), R and R¹ are each an aliphatic, olefinic, aromatic orarylaromatic radical having 2 to 30 carbon atoms, and optionallysubstituted with the following groups: hydroxyl, amino, alkoxide,cyanide, thiocyanide, halogen, sulfonic acid, sulfonic ester, thiol,benzoic acid, benzoic ester, carboxylic acid, carboxylic ester,acrylate, methacrylate or organosilane radical, it being possible for Rand R¹ to have an identical or different definition or substitution, nis 0, 1 or 2, Alk is a divalent unbranched or branched hydrocarbonradical having 1 to 6 carbon atoms, m is 0 or 1, Ar is an aryl radicalhaving 6 to 12 carbon atoms, preferably 6 carbon atoms, which can besubstituted by the following groups: hydroxyl, amino, alkoxide, cyanide,thiocyanide, halogen, sulfonic acid, sulfonic ester, thiol, benzoicacid, benzoic ester, carboxylic acid, carboxylic ester or organosilaneradical, p is 0 or 1, with the proviso that p and n are notsimultaneously 0, x is a number from 2 to 8, r is 1, 2 or 3, with theproviso that r+n+m+p=4, Alkyl is a monovalent unbranched or branchedunsaturated hydrocarbon radical having 1 to 20 carbon atoms, preferably2 to 8 carbon atoms, Alkenyl is a monovalent unbranched or branchedunsaturated hydrocarbon radical having 2 to 20 carbon atoms, preferably2 to 8 carbon atoms.
 8. A process for preparing a precipitated silicawherein the precipitated silica has a BET surface area in the range150-400 m²/g, a CTAB surface area in the range 145-350 m²/g, and anAl₂O₃ content in the range 0.2-5% by weight comprising, a) charging anaqueous waterglass solution into a reactor, b) metering waterglass andsulfuric acid into the reactor simultaneously into this initial chargeat from 55 to 95° C. for from 30 to 100 minutes with stirring forming amixture, c) acidifying the mixture with sulfuric acid to a pH of about 5to form a product, and d) filtering and drying the product, with theproviso that aluminum compounds are added in steps b) and/or c).
 9. Aprocess of claim 8, wherein the components supplied in steps b) and c)each have an identical or different concentration.
 10. A process ofclaim 8, wherein the components supplied in steps b) and c) each have anidentical feed rate.
 11. A process of claim 8, wherein the componentssupplied in steps b) and c) each have a different feed rate.
 12. Aprocess of claim 11, wherein with an identical concentration of thecomponents in steps b) and c) the feed rate in step c) is from 110 to200% of the feed rate in step b).
 13. A process of claim 11, whereinwith an identical concentration of the components in steps b) and c) thefeed rate in step c) is from 50 to 100% of the feed rate in step b). 14.A process of claim 8, wherein the drying is carried out by spin-flash,nozzle tower or spray drying and/or granulation with/without a rollcompactor.
 15. A process of to claim 8, wherein the precipitated silicais modified with organosilanes of the formula I to III in mixtures offrom 0.5 to 50 parts, based on 100 parts of precipitated silica, inparticular from 1 to 15 parts, based on 100 parts of precipitatedsilica, the reaction between precipitated silica and organosilane beingcarried out during the preparation of the mixture (in situ) orexternally by spray application and subsequent thermal conditioning ofthe mixture or by mixing of the silane and the silica suspension withsubsequent drying and thermal conditioning.
 16. A vulcanizable rubbermixture or vulcanizate comprising the precipitated silica of claim 1.17. A tire comprising a precipitated silica of claim
 1. 18. (canceled)19. A vulcanizable rubber mixture or vulcanizate comprising theprecipitated silica prepared according to claim
 8. 20. A tire comprisinga precipitated silica prepared according to claim
 8. 21. A batteryseparator, an anti-blocking agent, a flatting agent in a paint, a papercoating, a defoamer, a gasket, a keypad, a conveyor belt or a windowseal comprising the precipitated silica as claimed in claim 1.